1. Field of the Invention
The present invention relates to an improvement of active materials for positive electrodes in alkaline storage batteries.
2. Description of the Related Art
Since alkaline storage batteries, particularly small-sized sealed batteries, are superior to other battery systems in charge and discharge characteristics, cycle life and stability and reliability, and are well-balanced in these properties, they have considerably spread as main electric sources of various portable devices represented by mobile communication devices and personal computers. In addition, since they are also markedly excellent in charge and discharge characteristics and reliability as large electric sources, they are noticed as locomotive main electric sources such as for electric vehicles. Battery systems representing alkaline storage batteries are nickel-cadmium storage batteries which have been used for a long period of time, but nickel-metal hydride storage batteries in which a negative electrode of a hydrogen-storing alloy is used in place of cadmium negative electrode have been industrialized to attain a high energy density. Moreover, as to electrode supports, three-dimensional foamed nickel porous bodies of high porosity (more than 95%) filled with a nickel oxide at a high density (sponge metal nickel electrode: SME) have been industrialized in place of sintered electrodes to realize a high energy density. In this way, hitherto, with respect to alkaline storage batteries, investigation has been made mainly on miniaturization, saving in weight, and increase of energy density.
On the other hand, when application of alkaline storage batteries to electric vehicles, electric power tools and the like is considered, an important task is increase in output of batteries, namely, improvement of working voltage in high rate discharging. Furthermore, many of portable devices work under constant power discharging, and they are in the state of high rate discharging at the end of discharging at which discharge voltage decreases to cause further decrease of discharge voltage. Therefore, improvement of working voltage is also an important task in small-sized sealed alkaline storage batteries.
As a means for solving the task, JP-A-5-174867 proposes to add 0.3-3.5 N of at least one hydroxide selected from rubidium hydroxide and cesium hydroxide to an alkaline electrolyte. By the addition of these compounds, conductivity of the electrolyte is improved and rubidium ion and cesium ion act as catalysts, and thus the working voltage at high rate discharging can be increased. However, use of the expensive materials such as rubidium and cesium in actual batteries is difficult and has not yet been industrialized.
U.S. Pat. No. 5,567,549 (1996) discloses an active material for positive electrodes in alkaline storage batteries which comprises Ni(OH).sub.2 in which Al is dissolved to form a solid solution. This technique is such that when Al is dissolved in Ni(OH).sub.2 to form a solid solution, .alpha.-Ni(OH).sub.2 phase is stabilized and this .alpha.-phase containing multi-phase Ni(OH).sub.2 in which Al is dissolved in the state of solid solution performs a reaction of higher order, whereby capacity of the batteries can be increased.
It has been reported that .alpha.-like Ni(OH).sub.2 is obtained by replacing a part of Ni of Ni(OH).sub.2 with a metallic atom of higher valence such as Co(III), Mn(III), Fe(III) or the like ("Solid State Ionics", 32/33, p. 104(1989), "J. Power Sources", 35, p. 249(1991)). It has been considered that owing to the dissolution of a metallic atom of higher valence in solid state, the nickel plate layer in the crystal structure is positively charged and anion species (such as PO.sub.4.sup.3-, SO.sub.4.sup.2-, CO.sub.3.sup.2- and NO.sub.3.sup.-) enter between layers as charge compensation, resulting in a structure of the space between the layers being extended ("J. Power Sources", 36, p. 113(1991), "Materials Science Forum", 152-153, p. 201(1994)).
Further, improvement of charge and discharge characteristics by the dissolution of Co and Cd in Ni(OH).sub.2 in the state of solid solution is disclosed in Japanese Patent No.1827639 (1984) and JP-A-3-50384.
Furthermore, many reports have been made on the improvement of characteristics by coating treatment of active materials for positive electrodes. For example, JP-A-62-222566 and JP-A-62-234867 disclose that utilization ratio can be improved by coating the surface of particles of Ni(OH).sub.2 or those mainly composed of Ni(OH).sub.2 with a Co compound. JP-A-9-17428 reports that utilization ratio can be improved and discharge capacity can be increased by coating the surface of particles of Ni(OH).sub.2 or those mainly composed of Ni(OH).sub.2 with a Co and/or Ni additive. Moreover, JP-A-8-287907 discloses that alkaline storage batteries of high capacity and long life can be provided by using an active material for positive electrode prepared by coating the surface of particles of Ni(OH).sub.2 or those mainly composed of Ni(OH).sub.2 with a first compound layer mainly composed of a compound of Group II element and further coating the surface with a second compound layer mainly composed of a Co compound. In addition, JP-A-8-329943 discloses that alkaline storage batteries enhanced in utilization ratio, particularly utilization ratio at high temperatures can be provided by using an active material for positive electrodes which is prepared by dissolving Ca in Ni(OH).sub.2 particles in the state of solid solution and simultaneously coating and impregnating the surface and pores of the Ni(OH).sub.2 particles with at least a part of the contained Ca in the form of Ca(OH).sub.2. However, no report has been made on the improvement of discharge voltage and high rate discharge characteristics by coating treatment of the active material s for positive electrodes.